There are a variety of applications that require the use of a line image having a relatively uniform intensity. One such application is laser thermal processing (LTP), also referred to in the art as laser spike annealing (LSA), or just “laser annealing.” Laser annealing is used in semiconductor manufacturing for a variety of applications, including for activating dopants in select regions of devices (structures) formed in a semiconductor wafer when forming active microcircuits such as transistors.
One form of laser annealing uses a scanned line image from a light beam to heat the surface of the wafer to a temperature (the “annealing temperature”) for a time long enough to activate the dopants in the semiconductor structures (e.g., source and drain regions) but short enough to prevent substantial dopant diffusion. The time that the wafer surface is at the annealing temperature is determined by the power density of the line image, as well as by the line-image width divided by the velocity at which the line image is scanned (the “scan velocity”).
To achieve high wafer throughput in a commercial laser annealing system, the line image should be as long as possible, while also having a high power density. An example range for usable line-image dimensions is 5 mm to 100 mm in length (cross-scan direction) and 25 microns to 500 microns in width (scan direction). To achieve uniform annealing, it is also necessary for the intensity profile along the line-image length to be as uniform as possible, while non-uniformities along the line-image width are averaged out during the scanning process.
Typical semiconductor processing requirements call for the anneal temperature to be between 1000° C. and 1300° C., with a temperature uniformity of +/−3° C. To achieve this degree of temperature uniformity, the line image formed by the annealing light beam needs to have a relatively uniform intensity in the cross-scan direction, which under most conditions is less than a +/−5% intensity variation.
A CO2 laser is a preferred light source for laser annealing applications because its wavelength (nominally 10.6 microns) is much longer than the size of most device features on the wafer. This is important because using a wavelength on the order of the size of the device features can lead to pattern-related variations in exposure. Thus, when the wafer is irradiated with the 10.6 micron wavelength light, the light scattering from the features is minimal, resulting in a more uniform exposure. In addition, a CO2 laser emits a relatively high-intensity beam. However, the coherence length for a CO2 laser is relatively long, typically several meters. This makes it unfeasible to use a binary optic approach to produce a line image with the required degree of intensity uniformity, i.e., ˜10% (i.e., about +/−5%) based on principles of Kohler illumination.